feat(workspace): backlinks at the end of a note

GET /documents/{id}/backlinks lists the notes the user can open whose
blocks link [[Title]] (also |alias and #section, case-insensitive),
excluding the note itself and the trash. The collapsible "Verlinkt auf
diese Notiz" section shows each note with folder and the linking bullets;
a click jumps straight to the bullet.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/workspace/DocumentEditor.tsx

@@ -41,12 +41,14 @@ export interface DocumentEditorProps {
   onOpenShareModal: (doc: Document) => void;
   onSelectTag: (tag: string) => void;
   onWikiLinkClick?: (pageTitle: string) => void;
+  onOpenDocBlock?: (docId: string, blockId?: string) => void;
   onBackMobile?: () => void;
   onOpenTask?: (taskId: string) => void;
   onSaveStatusChange?: (status: DocSaveStatus, errorMsg?: string) => void;
 }
 
 import { EmojiPickerModal } from './EmojiPickerModal';
+import { BacklinksSection } from './BacklinksSection';
 
 function ensureNonEmptyBlocks(docId: string, blocks?: DocumentBlock[]): DocumentBlock[] {
   if (!blocks || blocks.length === 0) {
@@ -182,6 +184,7 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
   onDocMetadataChange,
   onDeleteDoc,
   onRestoreDoc,
+  onOpenDocBlock,
   onOpenShareModal,
   onSelectTag,
   onWikiLinkClick,
@@ -1070,6 +1073,8 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
               onUndo={handleUndo}
               onRedo={handleRedo}
             />
+
+            {onOpenDocBlock && !inTrash && <BacklinksSection docId={docId} title={title} onOpen={onOpenDocBlock} />}
           </>
         )}
       </div>